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  <div class="question_difficulty">
   难度：Medium
  </div>
  <div>
   <h1 class="question_title">
    5. Longest Palindromic Substring
   </h1>
   <p>
    Given a string
    <strong>
     s
    </strong>
    , find the longest palindromic substring in
    <strong>
     s
    </strong>
    . You may assume that the maximum length of
    <strong>
     s
    </strong>
    is 1000.
   </p>
   <p>
    <strong>
     Example 1:
    </strong>
   </p>
   <pre>
<strong>Input:</strong> "babad"
<strong>Output:</strong> "bab"
<strong>Note:</strong> "aba" is also a valid answer.
</pre>
   <p>
    <strong>
     Example 2:
    </strong>
   </p>
   <pre>
<strong>Input:</strong> "cbbd"
<strong>Output:</strong> "bb"
</pre>
  </div>
  <div>
   <h1 class="question_title">
    5. 最长回文子串
   </h1>
   <p>
    给定一个字符串
    <code>
     s
    </code>
    ，找到
    <code>
     s
    </code>
    中最长的回文子串。你可以假设&nbsp;
    <code>
     s
    </code>
    的最大长度为 1000。
   </p>
   <p>
    <strong>
     示例 1：
    </strong>
   </p>
   <pre><strong>输入:</strong> "babad"
<strong>输出:</strong> "bab"
<strong>注意:</strong> "aba" 也是一个有效答案。
</pre>
   <p>
    <strong>
     示例 2：
    </strong>
   </p>
   <pre><strong>输入:</strong> "cbbd"
<strong>输出:</strong> "bb"
</pre>
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